Scroll compressors operate by the rotation of one involute scroll relative to another, stationary scroll. Fluid pockets created between line contacting flanks of the scrolls are continually squeezed radially inwardly toward the central axis of the compressor housing. The compressed, pressurized refrigerant is forced through an outlet cut through the center of the end plate of the stationary scroll and into a sealed exhaust chamber formed between the underside of the scroll end plate and a rear head of the compressor housing. Within the exhaust chamber, the pressurized refrigerant is subject to pressure waves and pulsations that may cause undesirable noise levels, if not muffled. From the exhaust chamber, the pressurized refrigerant exits to a high pressure refrigerant line through a simple outlet, and eventually runs through a condenser.
Because the scroll itself requires a good deal of careful machining, it must be installed inside the housing separately. Most often, the scroll end plate is simply fixed into the housing by threaded bolts that run through the rear head and into the stationary scroll end plate. An example can be seen in U.S. Pat. No. 4,597,724. This is a simple securement technique, but leaves several potential leak paths out of the high pressure exhaust chamber at the points where the bolts pierce the rear head. Another design, shown in co-assigned U.S. Pat. No. 5,346,376, has a rear head that is molded integral to the cylindrical housing wall, and which is pierced only by the high pressure line outlet. The stationary scroll sits inside the housing wall on a cylindrical flange that holds it axially above the rear head. The stationary scroll is fixed to the flange by pins that prevent it from twisting, but which allow it to float axially, thereby adjusting the axial thrust pressure on seals that run along the tip of the scroll's involute wraps. This scroll securement technique is really only suitable for this specific compressor design, and not for a more conventional stationary scroll that is not designed to float axially.
Another means of securing the stationary scroll is to cast it integrally to and with the rear head. The rear head-stationary scroll unit is then fixed to the rest of the compressor housing, as by welding. An example may be seen in co-assigned U.S. Pat. No. 5,308,231. This design also theoretically avoids bolt holes through the rear head, since the stationary scroll is a permanent part of it. However, the casting process can be quite complex, especially if it is desired to place any pressure wave attenuation structures, such as baffles or sub-chambers.